In the United States and many other countries, heart disease is a leading cause of death and disability. One particular kind of heart disease is atherosclerosis, which involves the degeneration of the walls and lumen of the arteries throughout the body. Scientific studies have demonstrated the thickening of an arterial wall and eventual encroachment of the tissue into the lumen as fatty material builds upon the vessel walls. The fatty material is known as “plaque.” As the plaque builds up and the lumen narrows, blood flow is restricted. If the artery narrows too much, or if a blood clot forms at an injured plaque site (lesion), flow is severely reduced, or cut off and consequently the muscle that it supports may be injured or die due to a lack of oxygen. Atherosclerosis can occur throughout the human body, but it is most life threatening when it involves the coronary arteries which supply oxygen to the heart. If blood flow to the heart is significantly reduced or cut off, a myocardial infarction or “heart attack” often occurs. If not treated in sufficient time, a heart attack often leads to death.
The medical profession relies upon a wide variety of tools to treat coronary disease, ranging from drugs to open heart “bypass” surgery. Often, a lesion can be diagnosed and treated with minimal intervention through the use of catheter-based tools that are threaded into the coronary arteries via the femoral artery in the groin. For example, one treatment for lesions is a procedure known as percutaneous transluminal coronary angioplasty (PTCA) whereby a catheter with an expandable balloon at its tip is threaded into the lesion and inflated. The underlying lesion is re-shaped, and hopefully, the lumen diameter is increased to improve blood flow.
In recent years, a new technique has been developed for obtaining information about coronary vessels and to view the effects of therapy on the form and structure of a site within a vessel rather then merely determining that blood is flowing through a vessel. The new technique, known as Intracoronary/Intravascular Ultrasound (ICUS/IVUS), employs very small transducers arranged on the end of a catheter which provide electronic transduced echo signals to an external imaging system in order to produce a two or three-dimensional image of the lumen, the arterial tissue, and tissue surrounding the artery. These images are generated in substantially real time and provide images of superior quality to the known x-ray imaging methods and apparatuses. Imaging techniques have been developed to obtain detailed images of vessels and the blood flowing through them. An example of such a method is the flow imaging method and apparatus described in O'Donnell et al. U.S. Pat. No. 5,453,575, the teachings of which are expressly incorporated in their entirety herein by reference. Other imaging methods and intravascular ultrasound imaging applications would also benefit from enhanced image resolution.
Known intravascular ultrasound transducer assemblies have limited image resolution arising from the density of transducer elements that are arranged in an array upon a transducer assembly. Known intravascular transducer array assemblies include thirty-two (32) transducer elements arranged in a cylindrical array. While such transducer array assemblies provide satisfactory resolution for producing images from within a vasculature, image resolution may be improved by increasing the density of the transducer elements in the transducer array.
However, reducing the size of the transducer array elements increases the diffraction of the ultrasound beam emitted by a transducer element which, in turn, leads to decreased signal strength. For example, if the width of each of the currently utilized ferroelectric copolymer transducer elements is reduced by one-half so that sixty-four (64) transducer elements are arranged in a cylindrical array roughly the same size as the thirty-two (32) transducer array, the strength of the signal produced by the individual transducer elements in the sixty-four (64) element array falls below a level that is typically useful for providing an image of a blood vessel. More efficient transducer materials (having a lower “insertion loss”) may be substituted for the ferroelectric copolymer transducer material in order to provide a useful signal in an intravascular ultrasound transducer assembly having sixty-four (64) transducer elements in a cylindrical array. Such materials include lead zirconate titanate (PZT) and PZT composites which are normally used in external ultrasound apparatuses. However, PZT and PZT composites present their own design and manufacturing limitations. These limitations are discussed below.
In known ultrasound transducer assemblies, a thin glue layer bonds the ferroelectric copolymer transducer material to the conductors of a carrier substrate. Due to the relative dielectric constants of ferroelectric copolymer and epoxy, the ferroelectric copolymer transducer material is effectively capacitively coupled to the conductors without substantial signal losses when the glue layer thickness is on the order of 0.5 to 2.0 μm for a ferroelectric copolymer film that is 10–15 μm thick. This is a practically achievable glue layer thickness.
However, PZT and PZT composites have a relatively high dielectric constant. Therefore capacitive coupling between the transducer material and the conductors, without significant signal loss could occur only when extremely thin glue layers are employed (e.g. 0.01 μm for a 10–15 μm thick PZT transducer). This range of thicknesses for a glue layer is not achievable in view of the current state of the art.
Transducer backing materials having relatively low acoustic impedance improve signal quality in transducer assemblies comprising PZT or PZT composites. The advantages of such backing materials are explained in Eberle et al. U.S. Pat. No. 5,368,037 the teachings of which are expressly incorporated in their entirety herein by reference. It is also important to select a matching layer for maximizing the acoustic performance of the PZT transducers by minimizing echoes arising from the ultrasound assembly/blood-tissue interface.
Individual ferroelectric copolymer transducers need not be physically isolated from other transducers. However, PZT transducers must be physically separated from other transducers in order to facilitate formation of the transducers into a cylinder and to provide desirable performance of the transducers, such as minimization of acoustic crosstalk between neighboring elements. If the transducer elements are not physically separated, then the emitted signal tends to conduct to the adjacent transducer elements comprising PZT or PZT composite material.
Furthermore, the PZT and PZT composites are more brittle than the ferroelectric copolymer transducer materials, and the transducer elements cannot be fabricated in a solid flat sheet and then re-shaped into a cylindrical shape of the dimensions suitable for internal ultrasound imaging.
The integrated circuitry of known ultrasound transducer probes are mounted upon a non-planar surface. (See, for example, the Proudian '097 patent). The fabrication of circuitry on a non-planar surface adds complexity to the processes for mounting the integrated circuitry and connecting the circuitry to transmission lines connecting the integrated circuitry to a transmission cable and to the transducer array.
Yet another limitation on designing and manufacturing higher density ultrasound transducer arrays for intravascular imaging is the density of the interconnection circuitry between the ultrasound transducer elements and integrated circuits placed upon the ultrasound transducer assembly. Presently an interconnection density of about 0.002″ pitch between connection points is achievable using state-of-the-art fabrication techniques. However, in order to arrange sixty-four (64) elements in a cylindrical array having a same general construction and size (i.e., 1.0 mm) as the previously known 32 element array (e.g., the array disclosed in the Proudian et al. U.S. Pat. No. 4,917,097), the interconnection circuit density would have to increase. The resulting spacing of the interconnection circuitry would have to be reduced to about 0.001″ pitch. Such a circuit density is near the limits of current capabilities of the state of the art for reasonable cost of manufacturing.